Light’s speed limit is safe for now

News that the faster-than-light neutrinos that caused a sensation in September may have a mundane explanation – a faulty cable connection – comes as little surprise. There has never been a convincing physical explanation for the result.

In the weeks following the original announcement theoretical physicists wrote dozens of papers attempting to explain the results, many of which found their way to me. As an editor at the journal Physical Review Letters, my job is to consult experts in the field and ultimately decide which manuscripts to publish. I saw nothing to convince me that they had cracked it.

First, a quick refresher. The result came from the OPERA neutrino detector based at the Gran Sasso laboratory near L’Aquila, Italy. OPERA detected neutrinos fired from CERN, about 730 kilometres away in Geneva, Switzerland, and found they arrived 60 nanoseconds earlier than expected, implying they zipped along at one part in 40,000 faster than the speed of light.

The experiment was not conceptually difficult. It calculated the neutrinos’ speed as you would do for any moving object&colon; distance divided by time. However, the researchers had to precisely measure the neutrinos’ path through the Earth and synchronise the clocks at each end to an accuracy of a few parts per million. It now seems they made an error, though we are awaiting confirmation.

Another possibility still worth considering is that the result is a statistical fluke. Data are sometimes described as “statistically significant” if they reach the 95 per cent confidence level. This is a disquietingly lax standard. It doesn’t mean that there is a 95 per cent chance there is an effect. It means “supposing there is no effect, you will still get data showing one 5 per cent of the time”. Do hundreds of trials and you will see many spurious effects, even at the 99 per cent confidence level. Quite a number of experiments have disagreed with the standard model of particle physics at the 99 per cent confidence level but have fizzled out after more data was taken.

However, the OPERA team claim a confidence level greater than 99.9999999 per cent. In other words you would have to do billions of trials in order to expect to get this result by chance.

So why were physicists underwear-eating-sure that there must be a mundane explanation? Because, when it comes to data that conflicts with established theory, we tend to be Bayesians – we take our prior confidence in the established theory into account. As Andrew Pontzen of the University of Cambridge and Hiranya Peiris of University College London wrote in New Scientist in 2010&colon; “Bayesian statistics shows us that the anomalies in the data are insufficient on their own to motivate drastic revision.” We also need a plausible theory.

Sure enough, the theories soon started rolling in. In the first few weeks after the announcement, physicists produced more than a paper a day on the subject. Many put forward new models explaining how neutrinos could travel faster than light. The rest either tried to find flaws in the experimental techniques or pointed out constraints which rule out most of the new models.

In the first few weeks after the announcement physicists produced more than a paper a day

Many of these papers were submitted to Physical Review Letters. Thus far we have received over 50 papers and published just three, all in the last category (Physical Review Letters, vol 107, p 181803, p 241802 and p 251801).

All reach the same conclusion&colon; models which explain the result by breaking relativity are ruled out. It doesn’t matter how you do it, if you break relativity, your idea is a goner.

How so? The paper by Andrew Cohen of Boston University and his Nobel laureate colleague Sheldon Glashow (vol 107, p 181803) is the easiest to explain. Imagine an aeroplane going faster than the speed of sound. It creates a shockwave of air which you hear as a sonic boom, and which carries energy away. An analogous thing happens to a particle going faster than the speed of light&colon; it loses energy via a shockwave of particles. And yet the OPERA neutrinos arrived with no energy lost. So there was no shockwave and thus no faster-than-light travel.

Whenever a paper proposing a new model arrived on my desk, I would ask the authors to explain how they circumvented this constraint. None did so convincingly.

So even before the faulty-cable-connection news, boxer-betting Bayesians were resting easy. Now, they are likely to move on to something else.

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Robert Garisto is an editor at Physical Review Letters, published by the American Physical Society and based in Ridge, New York